User device beamforming training in wireless networks

ABSTRACT

A technique is provided for beamforming training, including: transmitting, by a network node, common control signaling via a plurality of beams periodically according to a predetermined beam sweeping pattern, and transmitting, via a set of one or more beams in each of one or more sequential time-domain resources, an aperiodic downlink sounding burst to at least one user device to allow the at least one user device to perform beamforming training.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage entry of PCT Application No.PCT/EP2015/077325 filed Nov. 23, 2015, entitled “USER DEVICE BEAMFORMINGTRAINING IN WIRELESS NETWORKS” which is hereby incorporated by referencein its entirety.

TECHNICAL FIELD

This description relates to communications.

BACKGROUND

A communication system may be a facility that enables communicationbetween two or more nodes or devices, such as fixed or mobilecommunication devices. Signals can be carried on wired or wirelesscarriers.

An example of a cellular communication system is an architecture that isbeing standardized by the 3^(rd) Generation Partnership Project (3GPP).A recent development in this field is often referred to as the long-termevolution (LTE) of the Universal Mobile Telecommunications System (UMTS)radio-access technology. E-UTRA (evolved UMTS Terrestrial Radio Access)is the air interface of 3GPP's Long Term Evolution (LTE) upgrade pathfor mobile networks. In LTE, base stations or access points (APs), whichare referred to as enhanced Node AP (eNBs), provide wireless accesswithin a coverage area or cell. In LTE, mobile devices, or mobilestations are referred to as user equipments (UE). LTE has included anumber of improvements or developments.

A global bandwidth shortage facing wireless carriers has motivated theconsideration of the underutilized millimeter wave (mmWave) frequencyspectrum for future broadband cellular communication networks, forexample. mmWave (or extremely high frequency) may, for example, includethe frequency range between 30 and 300 gigahertz (GHz). Radio waves inthis band may, for example, have wavelengths from ten to onemillimeters, giving it the name millimeter band or millimeter wave. Theamount of wireless data will likely significantly increase in the comingyears. Various techniques have been used in attempt to address thischallenge including obtaining more spectrum, having smaller cell sizes,and using improved technologies enabling more bits/s/Hz. One elementthat may be used to obtain more spectrum is to move to higherfrequencies, above 6 GHz. For fifth generation wireless systems (5G), anaccess architecture for deployment of cellular radio equipment employingmmWave radio spectrum has been proposed. Other example spectrums mayalso be used, such as cmWave radio spectrum (3-30 GHz).

SUMMARY

According to an example implementation, a method may includetransmitting, by a network node, common control signaling via aplurality of beams periodically according to a predetermined beamsweeping pattern; and transmitting, via a set of one or more beams ineach of one or more sequential time-domain resources, an aperiodicdownlink sounding burst to at least one user device to allow the atleast one user device to perform beamforming training.

According to another example implementation, an apparatus may include atleast one processor and at least one memory including computerinstructions, when executed by the at least one processor, cause theapparatus to: transmit, by a network node, common control signaling viaa plurality of beams periodically according to a predetermined beamsweeping pattern; and transmit, via a set of one or more beams in eachof one or more sequential time-domain resources, an aperiodic downlinksounding burst to at least one user device to allow the at least oneuser device to perform beamforming training.

According to another example implementation, a computer program productmay include a computer-readable storage medium and storing executablecode that, when executed by at least one data processing apparatus, isconfigured to cause the at least one data processing apparatus toperform a method including: transmitting, by a network node, commoncontrol signaling via a plurality of beams periodically according to apredetermined beam sweeping pattern; and transmitting, via a set of oneor more beams in each of one or more sequential time-domain resources,an aperiodic downlink sounding burst to at least one user device toallow the at least one user device to perform beamforming training.

According to another example implementation, an apparatus may includemeans for transmitting, by a network node, common control signaling viaa plurality of beams periodically according to a predetermined beamsweeping pattern; and means for transmitting, via a set of one or morebeams in each of one or more sequential time-domain resources, anaperiodic downlink sounding burst to at least one user device to allowthe at least one user device to perform beamforming training.

According to an example implementation, a method includes receiving, bya user device from a network node, common control signaling via aplurality of beams periodically according to a predetermined beamsweeping pattern; receiving, via a set of one or more beams in each of aplurality of sequential time-domain resources, an aperiodic downlinksounding burst; switching, by the user device, to a different receivebeam for each of the sequential time-domain resources; and selecting, bythe user device based on the received downlink sounding burst in each ofthe plurality of sequential time-domain resources, one or more beams touse for transmitting signals to or receiving signals from to the networknode.

According to another example implementation, an apparatus may include atleast one processor and at least one memory including computerinstructions, when executed by the at least one processor, cause theapparatus to: receive, by a user device from a network node, commoncontrol signaling via a plurality of beams periodically according to apredetermined beam sweeping pattern; receive, via a set of one or morebeams in each of a plurality of sequential time-domain resources, anaperiodic downlink sounding burst; switch, by the user device, to adifferent receive beam for each of the sequential time-domain resources;and select, by the user device based on the received downlink soundingburst in each of the plurality of sequential time-domain resources, oneor more beams to use for transmitting signals to or receiving signalsfrom to the network node.

According to another example implementation, a computer program productmay include a computer-readable storage medium and storing executablecode that, when executed by at least one data processing apparatus, isconfigured to cause the at least one data processing apparatus toperform a method including: receiving, by a user device from a networknode, common control signaling via a plurality of beams periodicallyaccording to a predetermined beam sweeping pattern; receiving, via a setof one or more beams in each of a plurality of sequential time-domainresources, an aperiodic downlink sounding burst; switching, by the userdevice, to a different receive beam for each of the sequentialtime-domain resources; and selecting, by the user device based on thereceived downlink sounding burst in each of the plurality of sequentialtime-domain resources, one or more beams to use for transmitting signalsto or receiving signals from to the network node.

According to another example implementation, an apparatus may includemeans for receiving, by a user device from a network node, commoncontrol signaling via a plurality of beams periodically according to apredetermined beam sweeping pattern; means for receiving, via a set ofone or more beams in each of a plurality of sequential time-domainresources, an aperiodic downlink sounding burst; means for switching, bythe user device, to a different receive beam for each of the sequentialtime-domain resources; and, means for selecting, by the user devicebased on the received downlink sounding burst in each of the pluralityof sequential time-domain resources, one or more beams to use fortransmitting signals to or receiving signals from to the network node.

The details of one or more examples of implementations are set forth inthe accompanying drawings and the description below. Other features willbe apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a wireless network according to an exampleimplementation.

FIG. 2 is a diagram of a wireless transceiver according to an exampleimplementation.

FIG. 3 is a diagram illustrating random access resources according to anexample implementation.

FIG. 4 is a diagram illustrating a transmission of common controlsignaling and a downlink sounding burst according to an exampleimplementation.

FIG. 5 is a diagram illustrating a transmission of common controlsignaling to one or more UEs via a plurality of beams according to apredetermined beam sweeping pattern according to an exampleimplementation.

FIG. 6 is a diagram illustrating initial access to a cell according toan example implementation.

FIG. 7 is a diagram illustrating a transmission of a downlink soundingburst as part of a handover procedure.

FIG. 8 is a flow chart illustrating operation of a network nodeaccording to an example implementation.

FIG. 9 is a flow chart illustrating operation of a user device/UEaccording to an example implementation.

FIG. 10 is a block diagram of a wireless station (e.g., basestation/access point or mobile station/user device) according to anexample implementation.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a wireless network 130 according to anexample implementation. In the wireless network 130 of FIG. 1, userdevices 131, 132, 133 and 135, which may also be referred to as mobilestations (MSs) or user equipment (UEs), may be connected (and incommunication) with an access point (AP), which may also be referred toas a base station (BS), an enhanced Node B (eNB) or a network node. Atleast part of the functionalities of an access point (AP), base station(BS) or (e)Node B (eNB) may also be carried out by any node, server orhost which may be operably coupled to a transceiver, such as a remoteradio head. AP 134 provides wireless coverage within a cell 136,including to user devices 131, 132, 133 and 135. Although only four userdevices are shown as being connected or attached to AP 134, any numberof user devices may be provided. AP 134 is also connected to a corenetwork 150 via a S1 interface 151. This is merely one simple example ofa wireless network, and others may be used.

A user device (user terminal, user equipment (UE)) may refer to aportable computing device that includes wireless mobile communicationdevices operating with or without a subscriber identification module(SIM), including, but not limited to, the following types of devices: amobile station (MS), a mobile phone, a cell phone, a smartphone, apersonal digital assistant (PDA), a handset, a device using a wirelessmodem (alarm or measurement device, etc.), a laptop and/or touch screencomputer, a tablet, a phablet, a game console, a notebook, and amultimedia device, as examples. It should be appreciated that a userdevice may also be a nearly exclusive uplink only device, of which anexample is a camera or video camera loading images or video clips to anetwork.

In LTE (as an example), core network 150 may be referred to as EvolvedPacket Core (EPC), which may include a mobility management entity (MME)which may handle or assist with mobility/handover of user devicesbetween BSs, one or more gateways that may forward data and controlsignals between the BSs and packet data networks or the Internet, andother control functions or blocks.

The various example implementations may be applied to a wide variety ofwireless technologies or wireless networks, such as LTE, LTE-A, 5G,cmWave, and/or mmWave band networks, or any other wireless network. LTE,5G, cmWave and mmWave band networks are provided only as illustrativeexamples, and the various example implementations may be applied to anywireless technology/wireless network.

FIG. 2 is a diagram of a wireless transceiver according to an exampleimplementation. Wireless transceiver 200 may be used, for example, at abase station (BS), e.g., Access Point (AP) or eNB, or other wirelessdevice. Wireless transceiver 200 may include a transmit path 210 and areceive path 212.

In transmit path 210, a digital-to-analog converter (D-A) 220 mayreceive a digital signal from one or more applications and convert thedigital signal to an analog signal. Upmixing block 222 may up-convertthe analog signal to an RF (e.g., radio frequency) signal. Poweramplifier (PA) 224 then amplifies the up-converted signal. According toan example implementation, the power amplifier may be integrated to orwith an antenna element. The amplified signal is then passed through atransmit/receive (T/R) switch (or Diplexer 226 for frequency divisionduplexing, to change frequencies for transmitting). The signal outputfrom T/R switch 226 is then output to one or more antennas in an arrayof antennas 228, such as to antenna 228A, 228B and/or 228C. Prior tobeing transmitted by one or more of the antennas in the array ofantennas 228, a set of beam weights V₁, V₂, . . . or V_(Q) is mixed withthe signal to apply a gain and phase to the signal for transmission. Forexample, a gain and phase, V₁, V₂, . . . or V_(Q), may be applied to thesignal output from the T/R switch 226 to scale the signal transmitted byeach antenna (e.g., the signal is multiplied by V₁ before beingtransmitted by antenna 1 228A, the signal is multiplied by V₂ beforebeing transmitted by antenna 2 228B, and so on), where the phase may beused to steer or point a beam transmitted by the overall antenna array,e.g., for directional beam steering. Thus, the beam weights V₁, V₂, . .. or V_(Q) (e.g., each beam weight including a gain and/or phase) may bea set of transmit beamforming beam weights when applied at or duringtransmission of a signal to transmit the signal on a specific beam, andmay be a set of receive beamforming beam weights when applied to receivea signal on a specific beam.

In receive path 212 of wireless transceiver 200, a signal is receivedvia an array of antennas 228, and is input to T/R switch 226, and thento low noise amplifier (LNA) 230 to amplify the received signal.According to an example implementation, the LNA may be co-located withan antenna element. The amplified signal output by LNA 230 is then inputto a RF-to-baseband conversion block 232 where the amplified RF signalis down-converted to baseband. An analog-to-digital (A-D) converter 234then converts the analog baseband signal output by conversion block 232to a digital signal for processing by one or more upperlayers/application layers.

Various example implementations may relate, for example, to 5G radioaccess systems (or other systems) with support for Massive MIMO(multiple input, multiple output) and optimized for operating in highcarrier frequencies such as cmWave frequencies (e.g. from 3 GHz onwards)or mmWave frequencies, as examples, according to an illustrative exampleimplementation. Those illustrative systems are typically characterizedby the need for high antenna gain to compensate for increased pathlossand by the need for high capacity and high spectral efficiency torespond to ever increasing wireless traffic. According to an exampleimplementation, the increased attenuation at higher carrier frequenciesmay, for example, be compensated by introducing massive (multi-element)antenna arrays and correspondingly antenna gain via beamforming at theaccess point (AP)/base station (BS) and/or user device. The spectralefficiency may typically improve with the number spatial streams thesystem can support and thus with the number of antenna ports at theAP/BS.

In an example implementation, A TX (transmit) beam may correspond to anantenna port. Typically, the needed TX beam resolution may depend on thenumber of antenna elements. For example, if the number of antennaelements is low, then the number of beams is also low, and if the numberof antenna elements is high, then there is a need for a higher number ofbeams, according to an illustrative example implementation.

According to an example implementation, techniques are described foruser device/UE beamforming training in a wireless network. Inillustrative example implementations, beamforming training may beperformed in a cellular wireless network with multiple user devices.According to an example implementation, a downlink beam search signalburst, which may be referred to as a downlink sounding burst, istriggered (caused to occur) or initiated based on one of a plurality oftriggers/triggering methods. The downlink sounding burst may allow auser device to perform beamforming training.

According to an example implementation, an access point/AP (or othernetwork node) may transmit common control signaling via each of aplurality of beams periodically according to a predetermined beamsweeping pattern. Common control signaling refers to control signalingtransmitted to all (or at least multiple) user device/UEs within a cellof wireless network. The common control signaling may provide adiscovery signal that is transmitted by the AP via the different beamsat predetermined time instants, according to an example implementation.For example, an access point may periodically transmit common controlsignaling for cell specific beams that allows user devices/UEs to detecta cell and corresponding cell specific beams.

According to an example implementation, the common control signalingthat is periodically transmitted via each of the plurality of beams mayinclude one or more of the following: 1) synchronization signals, suchas primary synchronization signals (PSS) and/or secondarysynchronization signals (SSS), which may be used by a user device todetermine symbol timing, frame timing and cell identity; 2) a physicalbroadcast control channel (PBCH) which may include, for example,parameters used for the initial access of the cell such as a downlinksystem bandwidth, a channel structure, cell specific beam transmissionstructure indicating how many cell specific beams cell are transmittedin total per one sweep period and how many cell specific beams aretransmitted in parallel, and/or part of a system frame number; and, 3)cell specific reference signals (CSI-RS signals) that may also identifya specific beam by including a beam ID, which may be provided as acombination of a timestamp plus an antenna port number (identifying anAP's antenna port associated with the beam), for example. Therefore, inan illustrative example implementation, the common control signaling mayinclude one or more of the synchronization signals, the physicalbroadcast control channel (PBCH) signals and/or the cell specificreference signals (CSI-RS) that may provide a beam ID or beamidentification information.

Each UE may receive the common control signaling via one or more of theperiodically transmitted beams from the AP or network node, and maydetermine a best set of one or more AP beams/antenna ports based on thecontrol channel signaling received via the one or more beams. Forexample, a UE may measure the cell specific reference signals (CSI-RS)received via one or more beams, and may determine the strongest or bestset of N AP/network node beams (e.g., the best set of N beamstransmitted by the AP) for the UE based on these signals. The UE maysignal the best set of N AP beams for the UE to the AP. The signalingmay be conveyed explicitly or implicitly (or a combination of them).

One of a plurality of triggers/triggering methods may cause the AP totransmit, via a set of one or more beams (e.g., via a set of the N bestAP beams identified by the user device) in each of a plurality ofsequential time domain resources, an aperiodic downlink sounding burst(e.g., which may be initiated/transmitted upon request/demand) to atleast one UE to allow the at least one UE to perform beamformingtraining for its receive (RX) and/or transmit (TX) beams. The beams inwhich the aperiodic downlink sounding burst is transmitted may be one ormore of the plurality of beams in which the common control signaling wastransmitted. In an example implementation, the UE may apply one or morereceive beams to receive the downlink sounding burst, in order todetermine the UE's best receive beam(s) for communicating with the AP.The wireless uplink (UL) and downlink (DL) channels between the AP andthe UE may typically be reciprocal. This reciprocal nature of thechannel may be true for various channel arrangements, such as, forexample, time division duplexing (TDD) mode where the wireless receiverand transmitter may operate on the same frequency band. There can alsobe significant channel correlation in at least certain properties of ULand DL band also in the case of frequency division duplexing (FDD) wherereceiver and transmitter operate on different frequency bands. Forexample, direction of arrival information (DoA) measured from downlink(DL) band may correlate well with the DoA in the uplink (UL) band.Therefore, by determining the UE's best set of one or more receive (RX)beams, the UE may use this information to determine the UE's best set oftransmit (TX) beams for transmitting to the AP/network node, forexample. However, in one embodiment, the TX and RX beams may not be thesame in all cases. For example, if the traffic is DL heavy, it may bebeneficial to aim at good angular resolution in DL side, e.g., in orderto maximize DL rank. Thus, the UE may be able to train both beamsseparately. For example, the transmit (TX) beams may be trained byforming receive (RX) beams to be as potential TX beams. In oneembodiment, the RX beams may be trained with different beamconfigurations (e.g., such as wider beams, narrower beams, etc.). Toprovide robustness for downlink control channel reception, and on theother hand to provide higher possibility for higher spatial multiplexingdegree in downlink, the UE may operate using wider RX beam width, whilefor uplink transmission the UE may use a narrower TX beam width toobtain enough high EIRP (equivalent isotropically radiated power), forexample. The network deployment may typically be dimensioned so thatdownlink link budget at cell edge can be met with omni-directional RXbeam(s).

Therefore, an AP may first transmit common control signaling via each ofa plurality of beams periodically according to a beam sweeping pattern.A UE may determine a best set of N (e.g., one or more) AP beams based onthe received signals and may notify the AP of the best set of N AP beams(beams applied by the AP to transmit the common control signaling). TheAP or network node, upon occurrence of a trigger, may schedule aplurality of (or one or more) sequential time domain resources. The APmay then transmit, via a set of (e.g., a set of N best beam(s)) of oneor more beams in each resource of the set of sequential time-domainresources, an aperiodic (e.g., upon demand/occurrence of a trigger)downlink sounding burst to the UE(s) to allow the UE(s) to performbeamforming training. Thus, the AP may repeatedly transmit the samedownlink sounding burst for the set of beam(s) via each scheduleddownlink resource within the set of sequential time-domain resources,e.g., to allow a UE(s) to test each of a plurality of receive beams, anddetermine a best receive beam(s) for receiving signalsfrom/communicating with the AP. In this manner, the UE may determine itbest receive beam(s) for receiving signals from the AP. And, based onthe best receive beam(s), the UE may also determine its best transmitbeam(s) for transmitting to the AP, e.g., due to the reciprocal natureof the UE→AP (uplink) wireless channel as compared to the AP→UE(downlink) wireless channel.

Various techniques may be used by the AP/network node to determine a setof one or more DL beams that the AP should use for transmitting thedownlink sounding burst to the UE. In a first illustrative example, theUE may signal or indicate a set of beams (e.g., best set of beams) tothe AP that the AP should use as TX beams for transmitting the DLsounding burst. In a second illustrative example, information may beprovided, e.g., implicitly within the random access preamble and relatedtiming, in order to identify a set of beams. More detailed informationrelated to the UE's best DL beam could be determined by the AP based onUL signal measurement (by AP). In yet another example implementation,the UE may transmit a signal, e.g., a random access (RACH) preamble withomnidirectional beam pattern or with random beamforming weights. The APmay then determine the direction of arrival of UE's signal (e.g.,direction with most received power) and the AP may then apply sequentialsweeping on that direction (or for the beam(s) for that direction).

There may be several different triggers/triggering methods that maycause the AP to transmit the downlink sounding burst(s) via sequentialtime-domain resources (e.g., via a plurality of sequential time slots),such as, for example: triggering (e.g., causing) the transmission of thedownlink sounding burst in response to a random access (RACH) procedure,such as, in response to the AP receiving a random access (RACH) preamblefrom a UE or receiving a connection request (e.g., RRC/Radio ResourceControl Connection Request, also known as msg 3 of a random accessprocedure); triggering or initiating the transmission of the downlinksounding burst in response to a handover procedure (e.g., in response toa handover notification or handover request from another AP/networknode); and, triggering or initiating the transmission of the downlinksounding burst in response to a channel state information (CSI)procedure (e.g., in response to channel state feedback from the UE)where a user device may send a CSI report/CSI feedback to the APreporting a state of a wireless channel. These are merely some exampletriggers/triggering methods that may trigger (cause) the transmission ofthe downlink sounding burst, and other triggers may be used to initiateor trigger an AP to transmit the downlink sounding burst(s). It may beup-to UE to decide whether there is a need for the downlink soundingburst(s). For example, in the case of digital UE architecture, there maybe no need for a separate DL sounding burst(s). On the other hand, theUE with hybrid architecture may have received sufficient amount ofreference signal to perform UE TX/RX beam training already. Finally, itmay be up-to AP to make the decisions whether to transmit the DLsounding burst(s) or not, and when.

For example, in response to measuring signal quality or signal strengthof cell specific reference signals (e.g., CSI-RS signals), a UE may senda CSI (channel state information) report to the AP indicating a rankindication (RI), a precoder matrix indication (PMI), that indicates apreferred precoder to use for downlink transmission, and a channelquality indication (CQI) that represents a modulation and coding scheme(MCS) that can be used for communication with the user device. The CSIreport may be sent by the UE to the AP and may also indicate a handoverfrom a source AP to a target AP, e.g., based on strength or quality ofsignals received from one or more APs.

Based on occurrence of one of a plurality of triggers/triggeringmethods, an AP/network node may schedule one or more or a plurality ofsequential (e.g., sequential in time) resources to transmit a downlinkburst of beam acquisition signals (downlink sounding bursts) to one ormore user devices to allow the user devices to perform beamformingtraining. The AP/network node may then transmit the downlink soundingburst via a set (e.g., subset of N best AP beams for one or more userdevices) of one or more beams in each of the plurality of sequentialtime-domain resources (e.g., sequential time slots). In an exampleimplementation, a UE may notify the AP of a set (one or more) of theAP's transmit beam(s) that should be used to transmit multiple instancesof the downlink sounding burst and a number of times/instances thedownlink sounding burst should be transmitted on sequential time-domainresources. For example, the UE may indicate that TX beams 1 and 2 (e.g.,out of 24 beams) from the AP are the best beams, and the user device mayrequest a repetitive transmission of the downlink sounding burst onthese AP transmit beams 1 and 2 for each of 4 time-domain resources(e.g., repeated transmission of downlink sounding burst for 4 sequentialtime slots/resources).

According to an example implementation, the transmission of the downlinksounding burst by the AP in (or during) each of the 4 sequentialtime-domain resources allows the UE to test/perform beamforming trainingwith the AP's transmitted beam 1 and/or beam 2 with 4 different receive(RX) beams of the UE, in order to determine the best receivebeam(s)/best antenna port(s) (e.g., the UE's beam(s) or antenna port(s)that produce a received signal having a best/greatest signal strength orsignal quality). Thus, for example, the UE may apply a different receivebeam when receiving the downlink sounding burst during each of the foursequential time-domain resources. The UE may then determine which of thefour applied receive beams/antenna ports (e.g., where there may be oneantenna port associated with each of a plurality of beams) provided thebest signal strength/signal quality as measured by the user device. Inan example implementation, a gap (in time) may be provided between eachsequential time-domain resource (or only a part of each time domainresource may be used by the AP to transmit the downlink sounding burst)to provide time to allow the UE to switch receive beams for each of thesequential time-domain resources.

Selection of AP beam(s)/antenna port(s) used for the transmission of adownlink sounding burst from the AP to one or more UEs may be based oninformation of which downlink beam(s)/antenna port(s) would be mostrelevant for the UE, e.g., the downlink sounding burst may betransmitted by the AP via one or more (up-to N) best beam(s)/antennaport(s) based on received information (best AP beam information) fromthe UE(s). In addition, the downlink sounding burst may be multiplexedwith downlink data symbols in a FDM (frequency division multiplex)manner. The downlink sounding burst may be triggered dynamically by theUE/user device.

In an example implementation, the transmission of the downlink soundingburst may be triggered as part of RACH (random access procedure/randomaccess channel) signaling transmitted by a UE (e.g., RACH/random accesspreamble or RACH/random access message 3, also known as a radio resourcecontrol/RRC connection request message). In some cases, a triggering of(or a message used to trigger) the sounding burst transmission mayindicate a number of transmission instances needed to performbeamforming training at the UE (e.g., indicating a number of sequentialtime-domain resources that should be scheduled and then used to transmitinstances of the sounding burst).

According to an example implementation, the UE may decide (or make thedecision or determination of) whether or not to trigger the transmissionof the downlink sounding burst. For example, as noted above, a UE maytransmit a message to the AP, such as: a random access preamble, arandom access procedure message 3/msg3 (RRC connection request), achannel state feedback, wherein receipt by the AP of any of thesemessages may trigger the AP to transmit the downlink sounding burst tothe UE. Triggering (or sending a message to trigger), of the downlinksounding burst by the UE, may also be based on perceived downlink signalquality and corresponding estimated required EIRP (effective isotropicradiated power) for the uplink transmission. For example, a UE locatedat or near a cell edge or in conditions where omni-directional antennapattern cannot be used may trigger the downlink sounding burst, while aUE being close to the AP may not need to trigger the downlink burst(e.g., since if the UE is near the AP, received signal quality/strengthat the UE may typically be sufficient without beamforming training). Thetriggering may be based also on the traffic conditions at the UE side,e.g., status of the UE's data buffer. The UE may also have up-to-datechannel state information (e.g., indicating that channel state/channelquality is sufficient, and beamforming training is not necessary for theUE) and a separate downlink sounding burst may not be needed.

Also, a type of transceiver architecture used/provided by the UE/userdevice may determine whether or not the downlink sounding burst isrequired and/or may determine a number of times the sounding burstshould be transmitted via sequential downlink resources. UEs based on orhaving a digital transceiver architecture may not require such adownlink sounding burst. Or, the downlink sounding burst may betransmitted via one downlink resource if the UE has a digitaltransceiver architecture. The AP may transmit the downlink soundingburst multiple times to the UE if the UE has/uses a hybrid transceiverarchitecture because beamforming training for multiple beams may berequired for hybrid (including both analog and digital portions of thetransceiver).

According to an example implementation, in a digital transceiverarchitecture, each transceiver unit (analog to digital conversion(A-D)+radio frequency (RF)+RF-to-baseband (BB) conversion, and digitalto analog conversion (D-A)+Upmixing-to-RF) is connected to one antenna.Also, for example, in a digital architecture, the UE can calculate inbaseband the TX beamforming weights based on received signals (alldirections/beams) from BS at once (no need to sequentially try differentRF beams but rather, the UE may process all possible directions at once)because in digital processing, all degrees of freedom/directions areavailable at one time. In a digital architecture, per antenna elementmeasurement can be made which means that UE may measure received signalsin all the directions from a single measurement.

On the other hand, in a hybrid architecture, there are multipletransceiver units (A-D+RF-to-BB-conversion/D-A+Upmixing-to-RF) whereeach transceiver unit is connected to multiple antennas that areconnected together with a RF/analog beamforming network. RF beamforminginvolved in hybrid architecture means that per antenna information isnot available at the UE receiver. When a UE would like to test differentbeams (directions) it requires that multiple measurement times areavailable and the UE measures CSI-RS with different beam patterns at atime.

According to an example implementation, a UE performing beamformingtraining for its RX and TX beams should be aware of the specific timingof the downlink sounding burst(s) so that UE can receive the downlinksounding burst(s). In a first example implementation, there may be afixed/predetermined timing relationship between a trigger (e.g.RACH/random access preamble) and the downlink sounding burst. Accordingto one illustrative example, a first downlink sounding burst may betransmitted by an AP 4 subframes after receipt by the AP of a randomaccess preamble. This is merely one example of a fixed timingrelationship for the downlink sounding burst, and other fixed timingrelationships may be used. According to another example implementation,where a selectable or variable timing relationship may be provided, theAP may schedule one or more resources for the transmission of thedownlink sounding burst(s), and may notify the UE of the scheduledresource(s) for the downlink sounding burst. For example, apredetermined Downlink Control Information (DCI) may be used forindicating the timing relationship (among other parameters) for thetransmitted downlink sounding burst(s). In one embodiment, differenttrigger methods may be associated with different fixed timingrelationships, e.g., a timing offset of 4 subcarriers may be associatedwith random access preamble acting as the trigger, whereas anothernumber of subframes may be associated with the CSI/channel statefeedback acting as the triggering method.

According to an example implementation, the transmitted downlinksounding burst may be the same signal (the same sounding burst)transmitted via multiple sequential/consecutive downlink resources.Also, a gap in time may be provided between consecutive resources, sothat a UE can perform RF (radio frequency) switching to a different beamduring the gap, e.g., so that the UE may apply a different receive beamto receive each instance of the downlink sounding burst, as part ofbeamforming training. The gap can be provided, e.g., such that there isone or more symbols, or at least a symbol fraction in betweenconsecutive transmission instances of a downlink sounding burst. Thisgap may be implemented, e.g., by one or more Zero-Tail DFT-S-OFDMAsymbols. Another option would be to use OFDMA symbols with zero-CP(switching is performed during CP/cyclic prefix).

FIG. 3 is a diagram illustrating random access resources according to anexample implementation. One or more of the random access resources shownin FIG. 3 may be used by a UE to transmit a random access preamble to anAP. According to an example implementation, the random access resourcesmay include a pool of non-triggering random access resources 310 (e.g.,including resources 312, 314, 316 and 318) or a pool of triggeringrandom access resources 320 (e.g., including resources 322, 324, 326 and328).

According to an example implementation, a random access preambletransmitted by a UE to an AP via one of the non-triggering resources 310will not trigger (e.g., does not cause or request) a transmission of adownlink sounding burst from the AP. On the other hand, a random accesspreamble transmitted by a UE to an AP via one of the triggering randomaccess resources 320 triggers (or requests or causes) a transmissionfrom the AP of the downlink sounding burst(s).

Furthermore, selection and use of a particular triggering resource(e.g., use of a particular one of triggering resources 322, 324, 326,328) may indicate or communicate a request by the UE to the AP totransmit a particular number of instances/repetitions of the downlinksounding burst to the UE. For example, a UE transmitting a random accesspreamble to an AP via triggering resource 322 indicates a request by theUE to the AP to transmit 2 instances (e.g., via 2 sequential time-domainresources) of the downlink sounding burst to the UE. Similarly, as shownin FIG. 3, triggering resources 324, 326 and 328 may be used by UE totransmit a random access preamble to an AP to request the AP to transmit4, 6 or 8, respectively, instances of the downlink sounding burst. Thenumbers 2, 4, 6 and 8 for triggering resources 322, 324, 326 and 328,respectively, are merely illustrative examples, and other numbers ofinstances/repetitions of the transmission of the downlink sounding burstmay be used.

In addition, a different triggering resource (e.g., 322, 324, 326, 328)may be used by a UE to transmit a random access preamble to indicate adifferent AP beam should be used for the transmission of the downlinksounding burst. For example, a UE may: transmit a random access preamblevia resource 322 to indicate that AP beam 1 should be used by the AP totransmit the downlink sounding burst, transmit a random access preamblevia resource 324 to indicate that AP beam 2 should be used by the AP totransmit the downlink sounding burst, transmit a random access preamblevia resource 326 to indicate that AP beam 3 should be used by the AP totransmit the downlink sounding burst, or may transmit a random accesspreamble via resource 328 to indicate AP beam 4 should be used by the APto transmit the downlink sounding burst.

FIG. 4 is a diagram illustrating a transmission of common controlsignaling and a downlink sounding burst according to an exampleimplementation. As shown in FIG. 4, the vertical (or Y) axis correspondsto different beams/beam IDs, from beam 1 to beam 13, in this example.The horizontal (or X) axis identifies time or time-domain resources,such as subframe or time-slot. Initially, at 410, during time-domainresource 1, the AP transmits common control signaling via beams 1-4.Similarly, during time-domain resources 13 and 26, the AP transmitscommon control signals via beams/beam IDs 5-8 and 9-12, respectively. Asa periodic process (the periodic transmission of common controlsignaling via one or more beams), the transmission of common controlsignaling via beams 1-4 repeats again during time-domain resource 39, asshown in FIG. 4.

According to this illustrative example, the UE may measure receivedchannel state reference signals received via the common controlsignaling received via each of a plurality of beams, and may determine,for example, that AP beam 6 (as an illustrative example) is thebest/strongest beam for the UE. The UE may determine or identify 4receive beams as the UE's best receive beams, and the UE may want toperform beamforming training for these 4 receive beams against a signalreceived from the AP via the AP transmit beam 6 (the AP's best transmitbeam with respect to the UE).

Therefore, according to an example implementation, at 420, the UEtransmits a trigger (e.g., a random access preamble, a random accessmessage 3/RRC connection request, a handover message/request, a channelstate information feedback, or other trigger or message to request orcause a transmission of downlink sounding burst(s)) to the AP duringresource 20, which may indicate or identify AP beam 6 for the downlinksounding burst, and may also indicate that 4 instances of the downlinksounding burst should be transmitted by the AP. For example, the UE mayuse a specific random access triggering resource to indicate the AP beam6 should be used for the downlink sounding burst and/or use a specificrandom access resource to indicate 4 instances of the downlink soundingburst. Alternatively, the AP may indicate, within one or more fields ofa trigger/message sent by the UE to the AP, the AP beam 6 and indicatethe 4 instances of the downlink sounding burst be transmitted to the UE.

At 430, in accordance with the trigger/sounding burst request receivedby the AP from the UE, the AP transmits the downlink sounding burst viabeam 6 for each of the requested number of (sequential) time-domainresources (4 sequential time-domain resources in this illustrativeexample), e.g., during time-domain resources 26-29, as shown in FIG. 4.According to an example implementation, in the case of the random accesspreamble triggering the downlink sounding bursts, the timing (or delay)of the transmission of the first downlink sounding burst at resource 26may occur a fixed number of resources (e.g., 6 resources in thisexample) after the transmission of the random access preamble (atresource 20). If a message (e.g., random access msg3, channel statefeedback, or other message) is transmitted by the UE to trigger thedownlink sounding burst, then this timing (or location) of the soundingburst(s) may be indicated in such message. In one example embodiment,the UE may first wait for all the common control signaling received viaall beams 1-12 before deciding which of the beams is best/are best forthis particular UE, e.g., based on signal strength or signal quality ofreceived signals.

A time gap may be provided between each resource used to transmit thesounding burst, e.g., to allow the UE to switch beams for each resource.Thus, the UE may receive the downlink sounding burst via AP beam 6during each of the 4 time-domain resources (resources 26, 27, 28 and 29in FIG. 4), and may apply a different receive beam for each of the 4resources in order to determine which of the 4 UE receive beams is thebest for receiving signals from the AP. This may be referred to asbeamforming training. In an example implementation, due to thereciprocal nature of the wireless channels, the UE may determine a besttransmit beam(s) for transmitting signals to the AP based on the best UEreceive beam(s). In this manner, the UE may perform beamforming trainingto select a receive beam and a transmit beam based on receiving adownlink sounding burst via one or more AP transmit beams for one ormore sequential time-domain resources.

FIG. 5 is a diagram illustrating a transmission of common controlsignaling to one or more UEs via a plurality of beams according to apredetermined beam sweeping pattern according to an exampleimplementation. In this simple illustrative example shown in FIG. 5, theAP may be able to transmit 6 different beams, and may have 2 antennaports. Based on the 2 antenna ports, the AP has the ability to transmit2 beams at a time. Thus, with the periodic transmission of the commoncontrol signaling, the AP may transmit the common control signaling(e.g., which may include synchronization signals, PBCH signals, CSI-RSsignals, and/or a beam ID/beam identification) via beam 1 and beam 2during time-domain resource t_(n). Similarly, the AP may transmit thecommon control signaling for beam 3 and beam 4 during time-domainresource t_(n+k), and may transmit common control signaling for beam 5and beam 6 during time-domain resource t_(n+k). Therefore, the beamsweeping pattern is that beams 1 and 2 are transmitted, followed bybeams 3 and 4, followed by beams 5 and 6. This process may then repeat,with transmission of common control signaling to multiple/all UEs forbeam 1 and beam 2.

The number used in the illustrative example shown in FIG. 5, e.g., 6total AP transmit beams and 2 antenna ports, and the example beamsweeping pattern, are merely illustrative examples, and other numbers,and other beam sweeping patterns may be used. For example, there may be48 different beams, and the AP may have 8 antenna ports (allowing the APto transmit 8 different beams at a time/simultaneously. Thus, in such acase, a periodic beam sweeping pattern may be used to transmit commoncontrol signaling may be used that transmits the common controlsignaling for all 48 beams during 6 sequential (or successive)time-domain resources as follows: beams 1-8, beams 9-16, beams 17-24,beams 25-32, beams 33-40, and beams 41-48 (and then repeat). This ismerely another example and is provided for illustration, and otherexamples may be used. Any number of total beams and antenna ports may beused.

As noted above, a selection of a RACH/random access resource mayindicate a request for a downlink sounding burst. The cell may havemultiple RACH/random access resources, which are divided into multipleresource pools, such as a pool of non-triggering resources 310 and apool of triggering resources 320 (see FIG. 3). For example, transmissionof a random access preamble via a resource in certain resource pool(s),e.g., via a resource within non triggering resources 310, does nottrigger a transmission of the downlink sounding burst from the AP to theUE, whereas a transmission of a random access preamble via a resource inother resource pool(s), e.g., via triggering resources 320, trigger (orcause) the downlink sounding burst to be transmitted from the AP to theUE.

According to an example implementation, resources that trigger thedownlink sounding burst may be further associated with downlink (APtransmit) beams (one-to-one mapping between a RACH/random accessresource and a downlink beam, or a one-to-many mapping between aRACH/random access resource and a set of downlink beams). According toan example implementation, a selection of certain RACH/random accessresources (e.g., selection of any of the resources 322, 324, 326, 328within triggering resources 320) may trigger correspondingly atransmission of a downlink sounding burst from the AP to the UE. Aselected RACH/random access resource used to transmit a random accesspreamble may also indicate to the AP a set of AP transmit (downlink)beam(s) to be used by the AP to transmit the downlink sounding burst tothe UE. According to an example implementation, information may bereceived from the UE (e.g., implicitly as part of RACH/random accesspreamble & related timing information), which may indicate a set ofbeams. More detailed information related to the best beam may bedetermined based on UL signal measurement (by AP), such as RACH/randomaccess preamble measurement, for example.

According to an example implementation, when a UE transmits a randomaccess preamble via a RACH/random access resource, the AP may respondwith a downlink control information (DCI) providing positive (ornegative) response for the preamble and scheduling information (e.g.,indicating the sequential time-domain resources when the sounding burstwill be transmitted) for the downlink sounding burst for TX beamtraining. An uplink grant for the first UE message containing data (suchas RACH Msg3/RRC connection request) may be signaled as well in the samedownlink control information (DCI) to the UE. Scheduling information forthe downlink sounding burst may be explicitly indicated in the DCI orderived implicitly from DCI (with predetermined rules).

Alternatively, there can be specific information elements (or fields)included in the RACH msg3/RRC connection request for triggering downlinksounding burst. By triggering a transmission of a downlink soundingburst via a RACH msg3/RRC connection request from a UE, this would allowmore bits to provide more information to the AP regarding the requestedsounding burst, such as identifying one or more AP transmit beams forthe burst, a number of instances (or a number of resources, or a numberof receive beams the UE would like to test) that the sounding burstshould be repeated via sequential time-domain resources, etc.Alternatively (or additionally), a trigger may be incorporated into aChannel State Information (CSI) acquisition procedure, e.g., wherein aUE may include a sounding burst trigger (e.g., indicating a request forsounding burst, identifying one or more AP TX beams, a number ofinstances to transmit the sounding burst) within a channel stateinformation (CSI) feedback/report sent to the AP. A UE may explicitlyindicate in the CSI report the need for beam re-adjustment procedure,which may cause the AP to transmit the downlink sounding burst(s).

In addition, according to another example implementation, an AP/networknode may trigger a transmission of a downlink sounding burst to a UEbased on a decrease in an uplink channel quality, for example. Also,according to an example implementation, an AP may trigger (or initiate)a transmission of downlink sounding burst based on load balancing amongits downlink beams that are linked to hardware resources (transceiverunits or antenna ports), e.g., based on a need to offload some trafficfrom a first beam/antenna port/transceiver to another beam/antennaport/transceiver, for example. Or, the AP may need to re-adjust certaindownlink beams (that also correspond to uplink RX beams at the UE).Therefore, the AP may trigger beamforming training based on adjusteddownlink AP beams. In addition, a handover of a UE from a source BS to atarget BS may trigger a transmission of the downlink sounding burst.According to an example implementation, information of target cell'sbest/strongest beams from UE point of view may be signaled to targetcell. The target cell may then schedule resources for transmission of adownlink sounding burst for TX beam training when the cell change occursfor the UE. Also, the target cell may also schedule uplink resources forthe UE, e.g. for the PRACH preamble and for timing acquisition in thenew cell.

According to an example implementation, the AP transmits periodicallyessential common control signaling in beam domain (cell specific beams)that allow UEs to detect a cell and corresponding cell specific beams.For example, the AP may transmit synchronization signal(s) and beamspecific reference signal from each cell specific beam. During onesweep, the AP covers the whole sector aperture with the beams. Dependingon applied beam widths in azimuth and elevation domain the number ofbeams may range from some tens up to hundred beams.

A hybrid UE relying on strong RF beamforming may have differentstrategies when doing initial cell search and subsequent initial access.A UE may start with omnidirectional RX beam pattern to perform cellsearch and cell selection resulting in lower complexity compared towhere a UE starts with narrow beams from the beginning (thisillustrative example may assume that the cells are dimensioned so thatomnidirectional RX beam at the UE can be used from downlink link budgetpoint of view).

When performing cell detection and selection based on periodical beambased discovery signal transmissions from an AP, a UE may be able tofind out the best or most relevant beams for itself. Typically, thenumber of strong AP (downlink) beams would be at maximum around 8 whilethe total number of downlink beams may be 32, 64, 96 or other number.Also, for UE TX beamforming training UE shall basically test all the RXbeam options against each downlink beam option. Thus, the complexity inUE TX beamforming training can be largely reduced when the BS transmitburst for the training only from the relevant downlink beams, and notfrom all the beams which would be the case if UE would start usingnarrow beams. To alleviate that, the UE may first find relevant downlinkbeams (e.g., which may be 2-8 AP beams, for example) usingomnidirectional RX pattern beam for which the UE would then requestseparate downlink sounding burst to train its RX/TX beams. As notedabove, these events may be incorporated into a RACH procedure, activedata transmission procedure and/or a handover procedure.

FIG. 6 is a diagram illustrating initial access to a cell according toan example implementation. A UE 132 and an AP/BS are shown in FIG. 6. At610, the AP 134 determines non-triggering resources 310, and triggeringresources 320 that may be used by UEs to transmit random accesspreamble. At 612, the AP 134 signals (or sends a message identifying)the random access resources, including the non-triggering resources 310and the triggering resources 320, for example. At 614, the UE 132determines a need for beamforming training, e.g., a need to receive adownlink sounding burst(s) from the AP 134 so that the UE 132 canperform beamforming training with respect to AP 134. At 616, UE 132selects a resource (e.g., resource 326, FIG. 3) of the triggeringresources 320. At 618, the UE 132 sends/transmits a random access/RACHpreamble to the AP 134 via the selected triggering resource 326, forexample. For example, a triggering resource may be used to transmit thepreamble that identifies a downlink/AP beam(s) for the sounding burstand/or a number of resources or a number of occurrences for transmissionof the sounding burst.

At 620, the AP 134 sends a positive response to the UE 132, includingscheduling information that identifies the resources (e.g., sequentialtime-domain resources) for transmission, via a set of one or beams, ofan aperiodic (e.g., transmitted on demand/request) downlink soundingburst. At 622, the AP 134 transmits, via a set of one or more APtransmit beams in each of the plurality of time-domain resources, anaperiodic downlink sounding burst to the UE 132 to allow the UE toperform beamforming training. At 622, the UE performs beamformingtraining based on the received occurrences of the downlink soundingburst, e.g., including applying a different receive beam to receive thesounding burst for each of the time-domain resources. The UE 132 maydetermine, e.g., based on signal strength or signal quality of thereceived sounding burst for each receive beam, which UE receive beam(s)is the best or strongest receive beam(s) for receiving signals from theAP 134. UE 132 may also determine a best transmit beam(s) fortransmitting to the AP 134, e.g., based on the best receive beam and thereciprocal nature of the uplink and downlink wireless channels betweenthe UE and AP. At 626, the UE 132 may send an uplink message to the AP134 using the identified best transmit beam.

According to an example implementation, a burst signal may be used thatmay use a waveform that has a built-in guard period for the beamswitching purposes (to allow the UE to switch beams between eachreceived sounding burst. One example is to use azero-tail-DFT-spread-OFDM waveform/symbol(s) to transmit the downlinksounding burst. Another option is to apply zero-CP OFDM symbol(s). Theseoptions allow RF beam switching within each symbol. This means that beamswitching can be fast. In the case of normal OFDMA, there, for example,may typically need to be a specific gap (such as a guard period)provided to allow for RF beam switching.

FIG. 7 is a diagram illustrating a transmission of a downlink soundingburst as part of a handover procedure. At UE 132 is shown, along with asource AP/BS and a target AP/BS. At 710, the UE 132 sends a measurementreport (e.g., identifying a signal strength of a target BS thatindicates a handover should be performed from source AP to target AP) tothe source AP. The measurement report may include a set of preferreddownlink/AP beams of the target AP, and/or a number of occurrences thatthe sounding burst should be transmitted from the target AP. At 712,handover preparation 712 is performed or exchanged between the source APand the target AP. As part of the handover preparation 712, source APmay signal to target AP: 1) a request to perform a handover for the UE132 to target AP, and 2) the set of preferred downlink/AP beams of thetarget AP that the UE 132 has identified for a sounding burst; and/or 3)the number of occurrences requested for transmission of the downlinksounding burst via the preferred set of downlink beams.

At 714, the target AP (AP of target cell) sends to the UE 132 thescheduling information that identifies the sequential resources fortransmission of the downlink sounding burst from target AP and an uplinkgrant for an initial uplink transmission from the UE to the target AP.This communication of scheduling information may typically occur via thesource cell. For example, the target AP may send to the UE, via thecurrent source AP (AP of the source cell), the scheduling informationthat identifies the sequential resources for transmission of thedownlink sounding burst and the uplink grant. At 716, the target AP maysend/transmit, via the set of the preferred downlink beam(s) in each ofthe plurality of identifies sequential resources, an aperiodic downlinksounding burst. At 718, the UE 132 performs beamforming training, e.g.,by applying a different UE receive beam to receive each of the downlinksounding bursts, and then determines which receive beam is the best forcommunicating with the target AP based on strength and/or quality of thereceived sounding bursts. The UE 132 then also determines a besttransmit beam (e.g., based on the best receive beam and the reciprocalnature of the uplink and downlink wireless channels) for transmitting tothe target AP. At 720, the UE 132 sends an uplink signal/message to thetarget AP via the best UE transmit beam for the target AP. The targetcell may also schedule uplink resources for the UE in sequent afterdownlink burst e.g. for the PRACH preamble and TA acquisition purpose inthe new cell.

FIG. 8 is a flow chart illustrating operation of a network nodeaccording to an example implementation. FIG. 8 may be directed tobeamforming training. Operation 810 includes transmitting, by a networknode, common control signaling via a plurality of beams periodicallyaccording to a predetermined beam sweeping pattern. And, operation 820includes transmitting, via a set of one or more beams in each of one ormore sequential time-domain resources, an aperiodic downlink soundingburst to at least one user device to allow the at least one user deviceto perform beamforming training.

According to an example implementation of the method of FIG. 8, thetransmitting an aperiodic downlink sounding burst may include:transmitting, via a set of one or more beams in each of a plurality ofsequential time-domain resources, an aperiodic downlink sounding burstto at least one user device to allow the at least one user device toperform beamforming training.

According to an example implementation of the method of FIG. 8, a numberof the one or more sequential time-domain resources for the transmissionof the downlink sounding burst is based on a number of transmit beams orreceive beams of the at least one user device. This relates to theangular resolution that can be achieved by the DL sounding burst, i.e.,how many different beam realizations can be covered.

According to an example implementation of the method of FIG. 8, a numberof the plurality of sequential time-domain resources for thetransmission of the downlink sounding burst is based on a number ofantenna elements of the at least one user device. For example, thenumber of antenna elements may correspond with the angular resolutionachievable (and needed). Thus, for example, a greater number of antennaelements may allow more beams and/or a finer resolution of beams.

According to an example implementation of the method of FIG. 8, aswitching gap is provided between each of the one or more sequentialtime-domain resources to allow the user device to switch to a differentreceive beam for each of the sequential time-domain resources.

According to an example implementation of the method of FIG. 8, whereinthe number of sequential resources is greater than 1 when the at leastone UE applies a hybrid transceiver architecture.

According to an example implementation of the method of FIG. 8, whereinthe number of sequential resources is equal to 1 when the at least oneUE applies a digital transceiver architecture.

According to an example implementation of the method of FIG. 8, whereinthe transmitting, via a set of one or more beams in each of one or moresequential time-domain resources, the aperiodic downlink sounding burstis triggered based on one of a plurality of triggering methods; andwherein a number of the one or more sequential time-domain resources forthe transmission of the downlink sounding burst is based on the one ofthe plurality of triggering methods.

According to an example implementation of the method of FIG. 8, whereinthe set of one or more beams used to transmit the aperiodic downlinksounding burst is derived based on at least one of a user devicetriggering that triggers the transmitting the downlink sounding burstand an uplink channel measurement by the network node.

According to an example implementation of the method of FIG. 8, whereinthe transmitting, via a set of one or more beams in each of one or moresequential time-domain resources, the aperiodic downlink sounding burstis triggered based on an occurrence of one of a plurality of triggeringmethods that include a first category of triggering methods and a secondcategory of triggering methods; wherein a number of the one or moresequential time-domain resources for the transmission of the downlinksounding burst is fixed for the first category of triggering methods,and is adjustable by the network node for the second category oftriggering methods.

According to an example implementation of the method of FIG. 8, whereinthe first category of triggering methods includes triggering thetransmitting of the aperiodic downlink sounding burst in response to arandom access preamble of a random access procedure; and wherein thesecond category of triggering methods comprises triggering or initiatingthe transmitting the aperiodic downlink sounding burst in response toreceiving at least one of the following: a handover message, a channelstate information feedback, and a subsequent message of the randomaccess procedure.

According to an example implementation of the method of FIG. 8, themethod further including transmitting scheduling information indicatingthe one or more sequential time-domain resources for the transmission ofthe aperiodic downlink sounding burst to the user device upon detectingan occurrence of the second category of triggering methods; and causinga transmission of the aperiodic downlink sounding burst to the userdevice according to the determined scheduling information.

According to an example implementation of the method of FIG. 8, themethod further including transmitting a list of a plurality of randomaccess resources that includes a non-triggering resource that is notassociated with triggering the transmission of the aperiodic downlinksounding burst, and a triggering resource that is associated withtriggering the transmission of the aperiodic downlink sounding burst;and, receiving, by the network node via the triggering resource, arandom access preamble; wherein the transmitting the aperiodic downlinksounding burst is triggered by the network node in response to receivingthe random access preamble via the triggering resource.

According to an example implementation of the method of FIG. 8, themethod further includes: determining scheduling information indicatingone or more sequential time-domain resources for the transmission of theaperiodic downlink sounding burst to the user device; transmitting thescheduling information to a network node of a source cell of a handoverprocess; and causing a transmission of the aperiodic downlink soundingburst to the user device according to the determined schedulinginformation after a handover has been performed for the user device.

According to an example implementation of the method of FIG. 8, whereinthe network node comprises a network node of a source cell, the methodfurther including: receiving, by the network node of the source cell, ahandover message indicating a handover of the user device to the networknode; receiving, by the network node of the source cell from a networknode of a target cell of the handover, scheduling information indicatingone or more resources of the target cell of the handover fortransmission of the downlink sounding burst; and sending, by the networknode of the source cell to the user device, the scheduling informationfor the transmission of the aperiodic downlink sounding burst by thenetwork node of the target cell.

According to an example implementation of the method of FIG. 8, whereinthe transmitting the aperiodic downlink sounding burst is initiated ortriggered based on an occurrence of one of a plurality of triggeringmethods; wherein there is a timing relationship between an occurrence ofthe triggering method and the transmission of the downlink soundingburst.

According to an example implementation of the method of FIG. 8, whereinthe timing relationship is fixed for a first category of triggeringmethods; and wherein the timing relationship is adjustable by thenetwork node for a second category of triggering methods.

According to an example implementation of the method of FIG. 8, themethod further including selecting, by the network node, a group of Nbest beams or antenna ports for the at least one user device based oninformation received from the at least one user device; and determining,based on the group of N best beams or antenna ports, the set of one ormore beams for transmitting, via the one or more sequential time-domainresources, the aperiodic downlink sounding burst.

According to an example implementation, an apparatus includes at leastone processor and at least one memory including computer instructions,when executed by the at least one processor, cause the apparatus to:transmit, by a network node, common control signaling via a plurality ofbeams periodically according to a predetermined beam sweeping pattern;and transmit, via a set of one or more beams in each of one or moresequential time-domain resources, an aperiodic downlink sounding burstto at least one user device to allow the at least one user device toperform beamforming training.

According to an example implementation, an apparatus includes: means(e.g., 1002A/1002B and/or 1004, FIG. 10) for transmitting, by a networknode, common control signaling via a plurality of beams periodicallyaccording to a predetermined beam sweeping pattern; and means (e.g.,1002A/1002B and/or 1004, FIG. 10) for transmitting, via a set of one ormore beams in each of one or more sequential time-domain resources, anaperiodic downlink sounding burst to at least one user device to allowthe at least one user device to perform beamforming training.

FIG. 9 is a flow chart illustrating operation of a user device/UEaccording to an example implementation. The method of FIG. 9 may bedirected to a method of beamforming training. Operation 910 includesreceiving, by a user device from a network node, common controlsignaling via a plurality of beams periodically according to apredetermined beam sweeping pattern. Operation 920 includes receiving,via a set of one or more beams in each of a plurality of sequentialtime-domain resources, an aperiodic downlink sounding burst. Operation930 includes switching, by the user device, to a different receive beamfor each of the sequential time-domain resources. And, operation 940includes selecting, by the user device based on the received downlinksounding burst in each of the plurality of sequential time-domainresources, one or more beams to use for transmitting signals to orreceiving signals from to the network node.

According to an example implementation of the method of FIG. 9, aswitching gap is provided between each of the plurality of sequentialtime-domain resources to allow the user device to switch to a differentreceive beam for each of the sequential time-domain resources.

According to an example implementation of the method of FIG. 9, themethod may further include receiving a list of a plurality of randomaccess resources that includes a non-triggering resource that is notassociated with triggering the transmission of the aperiodic downlinksounding burst, and a triggering resource that is associated withtriggering the transmission of the aperiodic downlink sounding burst;transmitting, by the user device to the network node via the triggeringresource, a random access preamble; wherein the receiving the aperiodicdownlink sounding burst comprises receiving, via a set of one or morebeams in each of a plurality of sequential time-domain resources, theaperiodic downlink sounding burst based on the transmission of therandom access preamble via the triggering resource.

According to an example implementation of the method of FIG. 9, themethod further including determining, by the user device, a group of Nbest beams or antenna ports of the network node based on the receptionof the common control signaling; and transmitting an indication of thegroup of N best beams or antenna ports to the network node.

According to an example implementation of the method of FIG. 9, themethod further including transmitting, by the user device, an indicationto the network node in order to instruct the network node to trigger atransmission of the aperiodic downlink sounding burst to the userdevice, wherein the indication is one of the following: a transmissionof a random access preamble, a transmission of a further message in arandom access procedure, a transmission of a channel state informationfeedback, and a transmission of a handover request.

According to an example implementation of the method of FIG. 9, whereinthere is a predetermined timing relationship between the transmitting ofthe indication and the receiving of the aperiodic downlink soundingburst.

According to an example implementation of the method of FIG. 9, themethod further including receiving scheduling information from thenetwork node after transmitting the indication, wherein the schedulinginformation indicates downlink resources for the reception, by the userdevice, of the aperiodic downlink sounding burst.

According to an example implementation, an apparatus includes at leastone processor and at least one memory including computer instructions,when executed by the at least one processor, cause the apparatus to:receive, by a user device from a network node, common control signalingvia a plurality of beams periodically according to a predetermined beamsweeping pattern; receive, via a set of one or more beams in each of aplurality of sequential time-domain resources, an aperiodic downlinksounding burst; switch, by the user device, to a different receive beamfor each of the sequential time-domain resources; and select, by theuser device based on the received downlink sounding burst in each of theone or more time-domain resources, one or more beams to use fortransmitting signals to or receiving signals from to the network node.

According to an example implementation, an apparatus includes means(e.g., 1002A/1002B and/or 1004, FIG. 10) for receiving, by a user devicefrom a network node, common control signaling via a plurality of beamsperiodically according to a predetermined beam sweeping pattern; means(e.g., 1002A/1002B and/or 1004, FIG. 10) for receiving, via a set of oneor more beams in each of a plurality of sequential time-domainresources, an aperiodic downlink sounding burst; means (e.g.,1002A/1002B and/or 1004, FIG. 10) for switching, by the user device, toa different receive beam for each of the sequential time-domainresources; and means (e.g., 1002A/1002B and/or 1004, FIG. 10) forselecting, by the user device based on the received downlink soundingburst in each of the one or more time-domain resources, one or morebeams to use for transmitting signals to or receiving signals from tothe network node.

According to an example implementation, a computer program product isprovided that includes a computer-readable storage medium and storingexecutable code that, when executed by at least one data processingapparatus, is configured to cause the at least one data processingapparatus to perform a method including: receiving, by a user devicefrom a network node, common control signaling via a plurality of beamsperiodically according to a predetermined beam sweeping pattern;receiving, via a set of one or more beams in each of a plurality ofsequential time-domain resources, an aperiodic downlink sounding burst;switching, by the user device, to a different receive beam for each ofthe sequential time-domain resources; and selecting, by the user devicebased on the received downlink sounding burst in each of the one or moretime-domain resources, one or more beams to use for transmitting signalsto or receiving signals from to the network node.

FIG. 10 is a block diagram of a wireless station (e.g., AP or userdevice) 1000 according to an example implementation. The wirelessstation 1000 may include, for example, one or two RF (radio frequency)or wireless transceivers 1002A, 1002B, where each wireless transceiverincludes a transmitter to transmit signals and a receiver to receivesignals. The wireless station also includes a processor or controlunit/entity (controller) 1004 to execute instructions or software andcontrol transmission and receptions of signals, and a memory 1006 tostore data and/or instructions.

Processor 1004 may also make decisions or determinations, generateframes, packets or messages for transmission, decode received frames ormessages for further processing, and other tasks or functions describedherein. Processor 1004, which may be a baseband processor, for example,may generate messages, packets, frames or other signals for transmissionvia wireless transceiver 1002 (1002A or 1002B). Processor 1004 maycontrol transmission of signals or messages over a wireless network, andmay control the reception of signals or messages, etc., via a wirelessnetwork (e.g., after being down-converted by wireless transceiver 1002,for example). Processor 1004 may be programmable and capable ofexecuting software or other instructions stored in memory or on othercomputer media to perform the various tasks and functions describedabove, such as one or more of the tasks or methods described above.Processor 1004 may be (or may include), for example, hardware,programmable logic, a programmable processor that executes software orfirmware, and/or any combination of these. Using other terminology,processor 1004 and transceiver 1002 together may be considered as awireless transmitter/receiver system, for example.

In addition, referring to FIG. 10, a controller (or processor) 808 mayexecute software and instructions, and may provide overall control forthe station 1000, and may provide control for other systems not shown inFIG. 8, such as controlling input/output devices (e.g., display,keypad), and/or may execute software for one or more applications thatmay be provided on wireless station 1000, such as, for example, an emailprogram, audio/video applications, a word processor, a Voice over IPapplication, or other application or software.

In addition, a storage medium may be provided that includes storedinstructions, which when executed by a controller or processor mayresult in the processor 1004, or other controller or processor,performing one or more of the functions or tasks described above.

According to another example implementation, RF or wirelesstransceiver(s) 1002A/1002B may receive signals or data and/or transmitor send signals or data. Processor 1004 (and possibly transceivers1002A/1002B) may control the RF or wireless transceiver 1002A or 1002Bto receive, send, broadcast or transmit signals or data.

The embodiments are not, however, restricted to the system that is givenas an example, but a person skilled in the art may apply the solution toother communication systems. Another example of a suitablecommunications system is the 5G concept. It is assumed that networkarchitecture in 5G will be quite similar to that of the LTE-advanced. 5Gis likely to use multiple input—multiple output (MIMO) antennas, manymore base stations or nodes than the LTE (a so-called small cellconcept), including macro sites operating in co-operation with smallerstations and perhaps also employing a variety of radio technologies forbetter coverage and enhanced data rates.

It should be appreciated that future networks will most probably utilizenetwork functions virtualization (NFV) which is a network architectureconcept that proposes virtualizing network node functions into “buildingblocks” or entities that may be operationally connected or linkedtogether to provide services. A virtualized network function (VNF) maycomprise one or more virtual machines running computer program codesusing standard or general type servers instead of customized hardware.Cloud computing or data storage may also be utilized. In radiocommunications this may mean node operations may be carried out, atleast partly, in a server, host or node operationally coupled to aremote radio head. It is also possible that node operations will bedistributed among a plurality of servers, nodes or hosts. It should alsobe understood that the distribution of labor between core networkoperations and base station operations may differ from that of the LTEor even be non-existent.

Implementations of the various techniques described herein may beimplemented in digital electronic circuitry, or in computer hardware,firmware, software, or in combinations of them. Implementations mayimplemented as a computer program product, i.e., a computer programtangibly embodied in an information carrier, e.g., in a machine-readablestorage device or in a propagated signal, for execution by, or tocontrol the operation of, a data processing apparatus, e.g., aprogrammable processor, a computer, or multiple computers.Implementations may also be provided on a computer readable medium orcomputer readable storage medium, which may be a non-transitory medium.Implementations of the various techniques may also includeimplementations provided via transitory signals or media, and/orprograms and/or software implementations that are downloadable via theInternet or other network(s), either wired networks and/or wirelessnetworks. In addition, implementations may be provided via machine typecommunications (MTC), and also via an Internet of Things (IOT).

The computer program may be in source code form, object code form, or insome intermediate form, and it may be stored in some sort of carrier,distribution medium, or computer readable medium, which may be anyentity or device capable of carrying the program. Such carriers includea record medium, computer memory, read-only memory, photoelectricaland/or electrical carrier signal, telecommunications signal, andsoftware distribution package, for example. Depending on the processingpower needed, the computer program may be executed in a singleelectronic digital computer or it may be distributed amongst a number ofcomputers.

Furthermore, implementations of the various techniques described hereinmay use a cyber-physical system (CPS) (a system of collaboratingcomputational elements controlling physical entities). CPS may enablethe implementation and exploitation of massive amounts of interconnectedICT devices (sensors, actuators, processors microcontrollers, . . . )embedded in physical objects at different locations. Mobile cyberphysical systems, in which the physical system in question has inherentmobility, are a subcategory of cyber-physical systems. Examples ofmobile physical systems include mobile robotics and electronicstransported by humans or animals. The rise in popularity of smartphoneshas increased interest in the area of mobile cyber-physical systems.Therefore, various implementations of techniques described herein may beprovided via one or more of these technologies.

A computer program, such as the computer program(s) described above, canbe written in any form of programming language, including compiled orinterpreted languages, and can be deployed in any form, including as astand-alone program or as a module, component, subroutine, or other unitor part of it suitable for use in a computing environment. A computerprogram can be deployed to be executed on one computer or on multiplecomputers at one site or distributed across multiple sites andinterconnected by a communication network.

Method steps may be performed by one or more programmable processorsexecuting a computer program or computer program portions to performfunctions by operating on input data and generating output. Method stepsalso may be performed by, and an apparatus may be implemented as,special purpose logic circuitry, e.g., an FPGA (field programmable gatearray) or an ASIC (application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer, chip orchipset. Generally, a processor will receive instructions and data froma read-only memory or a random access memory or both. Elements of acomputer may include at least one processor for executing instructionsand one or more memory devices for storing instructions and data.Generally, a computer also may include, or be operatively coupled toreceive data from or transfer data to, or both, one or more mass storagedevices for storing data, e.g., magnetic, magneto-optical disks, oroptical disks. Information carriers suitable for embodying computerprogram instructions and data include all forms of non-volatile memory,including by way of example semiconductor memory devices, e.g., EPROM,EEPROM, and flash memory devices; magnetic disks, e.g., internal harddisks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROMdisks. The processor and the memory may be supplemented by, orincorporated in, special purpose logic circuitry.

To provide for interaction with a user, implementations may beimplemented on a computer having a display device, e.g., a cathode raytube (CRT) or liquid crystal display (LCD) monitor, for displayinginformation to the user and a user interface, such as a keyboard and apointing device, e.g., a mouse or a trackball, by which the user canprovide input to the computer. Other kinds of devices can be used toprovide for interaction with a user as well; for example, feedbackprovided to the user can be any form of sensory feedback, e.g., visualfeedback, auditory feedback, or tactile feedback; and input from theuser can be received in any form, including acoustic, speech, or tactileinput.

Implementations may be implemented in a computing system that includes aback-end component, e.g., as a data server, or that includes amiddleware component, e.g., an application server, or that includes afront-end component, e.g., a client computer having a graphical userinterface or a Web browser through which a user can interact with animplementation, or any combination of such back-end, middleware, orfront-end components. Components may be interconnected by any form ormedium of digital data communication, e.g., a communication network.Examples of communication networks include a local area network (LAN)and a wide area network (WAN), e.g., the Internet.

While certain features of the described implementations have beenillustrated as described herein, many modifications, substitutions,changes and equivalents will now occur to those skilled in the art. Itis, therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the various embodiments.

What is claimed is:
 1. A method for beamforming training, comprising:transmitting, by a network node, common control signaling via aplurality of beams periodically according to a predetermined beamsweeping pattern; and transmitting, via a transmission beam and a set ofone or more receiving beams in each of one or more sequentialtime-domain resources, an aperiodic downlink sounding burst to at leastone user device to allow the at least one user device to performbeamforming training to select a downlink receive beam for receivingsignals from the network node.
 2. The method of claim 1 wherein a numberof the one or more sequential time-domain resources for the transmissionof the downlink sounding burst is based on a number of transmit beamsand/or receive beams of the at least one user device.
 3. The method ofclaim 1: wherein the transmitting, via the transmission beam and the setof one or more receiving beams in each of one or more sequentialtime-domain resources, the aperiodic downlink sounding burst istriggered based on one of a plurality of triggering methods; and whereina number of the one or more sequential time-domain resources for thetransmission of the downlink sounding burst is based on the one of theplurality of triggering methods.
 4. The method of claim 1: wherein thetransmission beam and the set of one or more receiving beams used totransmit the aperiodic downlink sounding burst are derived based on atleast one of a user device triggering that triggers the transmitting thedownlink sounding burst and an uplink channel measurement by the networknode.
 5. The method of claim 1: wherein the transmitting, via a set ofone or more beams in each of one or more sequential time-domainresources, the aperiodic downlink sounding burst is triggered based onan occurrence of one of a plurality of triggering methods that include afirst category of triggering methods and a second category of triggeringmethods; wherein a number of the one or more sequential time-domainresources for the transmission of the downlink sounding burst is fixedfor the first category of triggering methods, and is adjustable by thenetwork node for the second category of triggering methods.
 6. Themethod of claim 5: wherein the first category of triggering methodscomprises triggering the transmitting of the aperiodic downlink soundingburst in response to a random access preamble of a random accessprocedure; and wherein the second category of triggering methodscomprises triggering the transmitting the aperiodic downlink soundingburst in response to receiving at least one of the following: a handovermessage, a channel state information feedback, and a subsequent messageof the random access procedure.
 7. The method of claim 1, furthercomprising: transmitting a list of a plurality of random accessresources that includes a non-triggering resource that is not associatedwith triggering the transmission of the aperiodic downlink soundingburst, and a triggering resource that is associated with triggering thetransmission of the aperiodic downlink sounding burst; receiving, by thenetwork node via the triggering resource, a random access preamble;wherein the transmitting the aperiodic downlink sounding burst istriggered by the network node in response to receiving the random accesspreamble via the triggering resource.
 8. The method of claim 1, furthercomprising: transmitting scheduling information indicating the one ormore sequential time-domain resources for the transmission of theaperiodic downlink sounding burst to the user device upon detecting anoccurrence of the second category of triggering methods; and causing atransmission of the aperiodic downlink sounding burst to the user deviceaccording to the determined scheduling information.
 9. The method ofclaim 1, wherein the network node comprises a network node of a sourcecell of a handover, the method further comprising: receiving, by thenetwork node of the source cell, a handover message indicating ahandover of the user device to the network node; receiving, by thenetwork node of the source cell from a network node of a target cell ofthe handover, scheduling information indicating one or more resources ofthe target cell of the handover for transmission of the downlinksounding burst; and sending, by the network node of the source cell tothe user device, the scheduling information for the transmission of theaperiodic downlink sounding burst by the network node of the targetcell.
 10. The method of claim 1, wherein the network node comprises anetwork node of a target cell of a handover further comprising:determining scheduling information indicating one or more sequentialtime-domain resources for the transmission of the aperiodic downlinksounding burst to the user device; transmitting the schedulinginformation to a network node of a source cell of the handover; andcausing a transmission of the aperiodic downlink sounding burst to theuser device according to the determined scheduling information after thehandover has been performed for the user device.
 11. The method of claim1: wherein the transmitting the aperiodic downlink sounding burst isinitiated or triggered based on an occurrence of one of a plurality oftriggering methods; wherein there is a timing relationship between anoccurrence of the triggering method and the transmission of the downlinksounding burst.
 12. The method of claim 1, further comprising:selecting, by the network node, a group of N best beams or antenna portsfor the at least one user device based on information received from theat least one user device; and determining, based on the group of N bestbeams or antenna ports, the set of one or more beams for transmitting,via the one or more sequential time-domain resources, the aperiodicdownlink sounding burst.
 13. An apparatus comprising at least oneprocessor and at least one memory including computer instructions, whenexecuted by the at least one processor, cause the apparatus to: transmitcommon control signaling via a plurality of beams periodically accordingto a predetermined beam sweeping pattern; and transmit, via atransmission beam and a set of one or more receiving beams in each ofone or more sequential time-domain resources, an aperiodic downlinksounding burst to at least one user device to allow the at least oneuser device to perform beamforming training to select a downlink receivebeam for receiving signals from the apparatus.
 14. A computer programproduct, the computer program product comprising a non-transitorycomputer-readable storage medium and storing executable code that, whenexecuted by at least one data processing apparatus, is configured tocause the at least one data processing apparatus to perform the methodof claim
 1. 15. A method for beamforming training, comprising:receiving, by a user device from a network node, common controlsignaling via a plurality of beams periodically according to apredetermined beam sweeping pattern; receiving, via a transmission beamand a set of one or more receiving beams in each of a plurality ofsequential time-domain resources, an aperiodic downlink sounding burst;and performing beamforming training based on the aperiodic downlinksounding burst to select a downlink receive beam for receiving signalsfrom the network node.
 16. The method of claim 15, further comprising:receiving a list of a plurality of random access resources that includesa non-triggering resource that is not associated with triggering thetransmission of the aperiodic downlink sounding burst, and a triggeringresource that is associated with triggering the transmission of theaperiodic downlink sounding burst; transmitting, by the user device tothe network node via the triggering resource, a random access preamble;wherein the receiving the aperiodic downlink sounding burst comprisesreceiving, via a set of one or more beams in each of a plurality ofsequential time-domain resources, the aperiodic downlink sounding burstbased on the transmission of the random access preamble via thetriggering resource.
 17. The method of claim 15, further comprising:determining, by the user device, a group of N best beams or antennaports of the network node based on the reception of the common controlsignaling; and transmitting an indication of the group of N best beamsor antenna ports to the network node.
 18. The method of claim 15,further comprising: transmitting, by the user device, an indication tothe network node in order to instruct the network node to trigger atransmission of the aperiodic downlink sounding burst to the userdevice, wherein the indication is one of the following: a transmissionof a random access preamble, a transmission of a further message in arandom access procedure, a transmission of a channel state informationfeedback, and a transmission of a handover request.
 19. The method ofclaim 15, further comprising: receiving scheduling information from thenetwork node after transmitting the indication, wherein the schedulinginformation indicates downlink resources for the reception, by the userdevice, of the aperiodic downlink sounding burst.
 20. A computer programproduct, the computer program product comprising a non-transitorycomputer-readable storage medium and storing executable code that, whenexecuted by at least one data processing apparatus, is configured tocause the at least one data processing apparatus to perform a methodcomprising: receiving, by a user device from a network node, commoncontrol signaling via a plurality of beams periodically according to apredetermined beam sweeping pattern; receiving, via a transmission beamand a set of one or more receiving beams in each of a plurality ofsequential time-domain resources, an aperiodic downlink sounding burst;and performing beamforming training based on the aperiodic downlinksounding burst to select a downlink receive beam for receiving signalsfrom the network node.
 21. An apparatus comprising at least oneprocessor and at least one memory including computer instructions, whenexecuted by the at least one processor, cause the apparatus to: receive,from a network node, common control signaling via a plurality of beamsperiodically according to a predetermined beam sweeping pattern;receive, via a transmission beam and a set of one or more receivingbeams in each of a plurality of sequential time-domain resources, anaperiodic downlink sounding burst; and perform beamforming trainingbased on the aperiodic downlink sounding burst to select a downlinkreceive beam for receiving signals from the network node.